Ultrasonic transducer design is complex because material properties, structures geometries, assembly processes, as well as electrical connections, analog electronics front ends and digital computation algorithms have to be optimized in order to fit the specific requirements of the applications. In particular the size, electro-acoustic transmission and reception efficiency, operating frequency and bandwidth of the ultrasonic transducer are parameters of great importance depending on the ultrasonic transducer application. As an example, for imaging applications, spatial resolution is of prime importance. This leads to the selections of small size focused high frequency ultrasonic transducers with high damping in order to optimize spatial resolution both in lateral and depth scanning. For that reason ultrasonic transducer performance is a notion relative to the application fields. This is especially true for measurements in harsh downhole environments such as downhole oil and gas wells where equipments are required to operate under high temperature, high pressure, high shocks and vibrations and are submitted to corrosive fluids.
A typical ultrasonic transducer has a layer structure comprising a piezoelectric element, a backing part at a back side of the piezoelectric element and an impedance matching part at a front side of the piezoelectric element. The backing part is generally made of metal particles filled rubber that provides optimal damping and acoustic impedance characteristics, as well as electrical connection for the ultrasonic transducer. Such an ultrasonic transducer cannot be used in downhole environment where high pressure, high temperature and mechanical shocks would induce stresses resulting in damages to the piezoelectric element.
The oilfield industry has developed specific ultrasonic transducers allowing operation at high pressure, high temperature and protecting the ultrasonic transducers against conductive and corrosive fluids.
According to a first solution, the typical ultrasonic transducer hereinbefore described is positioned inside a compensated pressure chamber filled with a liquid (e.g. oil). The document U.S. Pat. No. 7,513,147 (FIG. 1A) describes an acoustic sensor 1 for use in a downhole measurement tool including a piezo-composite transducer element 2. In various exemplary embodiments, the acoustic sensor further includes a composite backing layer 3, at least one matching layer 4, and a barrier layer 5 deployed at an outermost surface of the sensor. The sensor comprises an annular region including a pressure equalization layer 6 disposed inside the housing 7 and around the sensor components. The pressure equalization layer 6 is a thin layer of silicone oil and functions to evenly distribute borehole pressure changes about the sensor components. Exemplary embodiments of this invention may withstand the extreme temperatures, pressures, and mechanical shocks frequent in downhole environments and thus may exhibit improved robustness. Also, the document US 2007/084277 describes an ultrasonic sensor placed in a wellbore having a resonant member that is exposed to a fluid in the wellbore. At a location in the wellbore, acoustic energy is measured wherein the acoustic energy is related to turbulence from formation fluid entering the wellbore. The ultrasonic sensor generates electrical signals when exposed to ultrasonic turbulences caused by a formation fluid entering into the wellbore. A processor processes the electrical signals to detect the flow of the formation fluid entering into the wellbore. Such an ultrasonic sensor comprises a pressure compensation arrangement to compensate for the pressure variations in the wellbore.
According to a second solution, the typical ultrasonic transducer 1 hereinbefore described further comprises elements made of flexible materials that absorb displacements induced by high pressure and high temperature. The document EP 2 610 432 (FIG. 1B) describes an ultrasonic transducer for use in a downhole measurement tool. The ultrasonic transducer 1 includes a piezoelectric element 2 coupled to a backing 3, and enclosed within a compliant material housing 7. The compliant material housing 7 may be flexible for withstanding extreme temperatures, pressures, and mechanical shocks that may be present in downhole environments.
The drawbacks of such conventional solutions are their complex design using expensive and large mechanical pieces, hermetic electrical feedthrough to connect the transducer to electronic circuits which are positioned in part of the tool at atmospheric pressure, external membrane (e.g. of fluoroelastomer) to protect the piezo element against corrosive and conductive fluids, or complex pressure compensation arrangement to compensate for the pressure variations in the wellbore.